Lithium secondary batteries have a higher energy density in theory compared to other batteries and thus allow to manufacture a small and light-weight battery. Therefore, vigorous studies have been focussed thereon to develop a power source of portable electronic instruments. Particularly, performance of such instruments is even increasing in recent years and their power source is required concominantly therewith to exhibit better discharging characteristics even at a high load. In order to fulfill these requirements, various studies are in progress next to the prior art battery using nonaqueous electrolyte solutions referred to as lithium ion battery to develop a battery using a polymer electrolyte that functions both as the nonaqueous electrolyte solution and the polymer separator of the prior art battery. Much interest has been focussed to a lithium secondary battery using the polymer electrolyte because of its remarkable advantages such as the possibility of making the battery smaller and thinner in size and lighter in weight as well as leak free.
Generally, secondary batteries now available in the market such as lithium secondary batteries make use of a nonaqueous electrolyte solution prepared by dissolving an electrolyte salt in an organic solvent. The use of this solution is problematic because the solution is easily susceptible to leakage from the battery parts, dissolution of electrode substances or vaporization which may develop problems of long term reliability, spilling off in the sealing process and the like.
In order to improve these problems, lithium secondary batteries have been developed which make use of a polymer electrolyte macroscopically occurring as a solid. The polymer electrolyte consists of a porous matrix of an ion-conductive polymer impregnated with or retaining a nonaqueous electrolyte solution (a lithium salt solution in an aprotic polar organic solvent).
Carbonaceous materials have also been studied and used in practice as an anode material in recent years because their potential at which they include and release lithium is much closer to the potential at which lithium precipitates and dissolves than other electrode materials.
Graphite is a carbonaceous material having a high capacity per unit weight and unit volume among a large number of carbonaceous materials because graphite is capable of inclusion of a single lithium atom per every 6 carbon atoms within its crystal lattice in theory. Graphite exhibits a generally flat lithium inclusion and release potential and is chemically stable. These properties largely contribute to the battery cycle stability.
Graphite-based carbonaceous materials give a discharge capacity close to the theoretical capacity in a nonaqueous electrolyte solution primarily containing ethylene carbonate as noted above. However, a problem remains to exist that its high crystallinity tends to cause decomposition of the nonaqueous electrolyte solution. For example, propylene carbonate (PC) used as a solvent has a wide potential window, a low solidifying temperature (−70° C.) and a high chemical stability and, therefore, has been widely used as a solvent in the nonaqueous electrolyte solution of the lithium battery. However, it is reported that when graphite is used as an anodic electroactive substance, PC is remarkably decomposed and the charge and discharge of the graphite anode becomes impossible even when PC is present in the electrolyte solution only at 10%. See, J. Electochem. Sco., Vol. 142, 1746(1995).
In order to improve the ion-conductivity at a low temperature, a variety of nonaqueous electrolyte solutions using a mixture of EC with various low-viscosity solvents have been reported considering difficult penetration of the electrolyte solution into the electrode when the solvent thereof is EC alone having a high viscosity. However, certain problems remain unsolved in the steps of manufacturing the battery such as volatility and leakage of the solvent. Although the use of macroscopically solid polymer electrolyte in conjunction with the carbonaceous anode material may overcome many of the above problems associated with the prior art batteries, certain new problems arising out from the use of polymer electrolyte and carbonaceous anode have been found.
The polymer electrolyte is prepared by polymerizing and crossling a precursor monomer of the ion-conductive polymer in a mixture with the nonaqueous electrolyte solution in situ. Whether the polymerization is heat polymerization or photopolymerization, it is imperative to use a polymerization initiator. Because not all amounts of the monomer and the initiator have been consumed in the polymerization reaction, it is inevitable for the monomer and the initiator to remain in the resulting polymer electrolyte at a certain level.
The presence of residual monomer can develop a problem of formation of a passivation film on the electrode that increases interfacial resistance between the electrode and the polymer electrolyte and also evolution of a gas as a consequence of the chemical reaction of its polymerizable carbon to carbon double bond with the cathodic and/or anodic electroactive substances. The same applies to residual polymerization initiators. Since residual monomer and initiator may adversely affect the battery performance, e.g. charge-discharge cycle characteristics and discharging characteristics at high load in particular, it is desirable to decrease the residual monomer and initiator levels as low as possible.
JP-A-10218913 teaches that the amount of unreacted monomer and polymerizable oligomers in the polymer electrode may be decreased to lower than 30% by weight by irradiating the monomer with UV radiation at an intensity greater than 20 mW/cm2. This method intends to elevate the reaction rate of the monomer by means of polymerization conditions and does not relates to the reduction of residual monomer levels secondarily after polymerization. Moreover the method does not eliminate nor ameliorate the adverse effects of residual initiator on the battery performance.
JP-A-10-158418 teaches to decrease the residual initiator and stabilizer levels in a self-sustained film of ion-conductive polymer by heating the film at an elevated temperature, for example, at 150° C. or ultrasonically rinsing the film in tetrahydrofuran. The treated film is electrolyte-free and, therefore, must be impregnated with a nonaqueous electrolyte solution.
This method is also disadvantageous compared with the method in which a gelled polymer electrolyte layer is formed integrally with the electrode by casting a monomer-nonaqueous electrolyte solution mixture on the electroactive substrate layer of the respective electrodes and then irradiating with UV radiation not only in terms of production efficiency due to a large number of steps but also in terms of risks of adverse effects on the battery performance due to insufficient impregnation and electrical and mechanical malcontact of the film with the electrode.
Accordingly, the problems to be solved by the present invention is to overcome the disadvantages of the prior art batteries and also to eliminate or ameliorate any adverse effect of residual monomer and initiator in the polymer electrolyte on the battery performance.